The invention may be used in molding plastic material which is introduced in a mold by injection or in a blow-mold when making hollow articles. The mold is being cooled for solidifying the plastic material of which at least the outer layer shall be temporarily heated up before or during the cooling step.
For example, extrusion blown articles receive a glossy surface, when the plastic material after being brought into contact with the inner surface of the mold is temporarily heated at its surface so that a thin outer layer comes to melting whereupon the surface subsequently participates again in the cooling step to solidify the material.
As a further example, in processing polymers capable of crystallization the polymer may be heat set by a temporary heating step. The article blow-molded exhibits a relatively thin wall thickness. According to the prior art the material subjected to the blow pressure and contacting the inner surface of the blow-mold is heated throughout for some time until crystallization takes place. Subsequently, the material is cooled to obtain stability.
In an injection molding process it can be helpful to heat narrow passages of the form during the filling step. These passages must be then cooled when the material injected is solidified.
A variety of devices is known to perform a heating step followed by cooling blow-molded articles in a rather short time and with little loss of energy. For example, a blow-mold may be provided with internal passages to alternatively receive a hot and a chilled liquid. Further, the blow-mold may be provided with separate heating and cooling passages. As a further example of the prior art a blow-mold may be lined by a pair of thin walled shells which are separate with respect to the blow-mold body to allow the shells to become rapidly cooled and heated.